Bob, is this something I could possibly be able to use in this application?
Where does one find such high-impedance coax? As an alternative, would it
be possible to put an active device up near the antenna that would act as a
buffer to feed a low-impedance line?

Dave


"Bob Lewis (AA4PB)" wrote in message
...
If the antenna is the typical end-fed wire then the impedance will be
very high. Most AM radios have a high impedance input designed to
match a random wire like that. If you use coax, the capacitance of the
coax (center conductor to shield) will essentially short most of the
signal to ground and little will get to the receiver. If you've ever
opened up the coax used on car radios you'll find that it is a special
high impedance coax. A small wire fits loosly thru an insulated tube
with the shield around the outside. This keeps a relatively large
spacing between the center conductor and the shield in order to
minimize capacitance.

David Forsyth wrote:
Bob, is this something I could possibly be able to use in this application?
Where does one find such high-impedance coax? As an alternative, would it
be possible to put an active device up near the antenna that would act as a
buffer to feed a low-impedance line?

Dave

The trick in this recommendation is that the car radio feedline is a
specific length (a few feet)and its capacitance is taken into account.

-Bill




"Bob Lewis (AA4PB)" wrote in message
...

If the antenna is the typical end-fed wire then the impedance will be
very high. Most AM radios have a high impedance input designed to
match a random wire like that. If you use coax, the capacitance of the
coax (center conductor to shield) will essentially short most of the
signal to ground and little will get to the receiver. If you've ever
opened up the coax used on car radios you'll find that it is a special
high impedance coax. A small wire fits loosly thru an insulated tube
with the shield around the outside. This keeps a relatively large
spacing between the center conductor and the shield in order to
minimize capacitance.

David Forsyth wrote:
Bob, is this something I could possibly be able to use in this application?
Where does one find such high-impedance coax? As an alternative, would it
be possible to put an active device up near the antenna that would act as a
buffer to feed a low-impedance line?

Dave

The trick in this recommendation is that the car radio feedline is a
specific length (a few feet)and its capacitance is taken into account.

-Bill




"Bob Lewis (AA4PB)" wrote in message
...

If the antenna is the typical end-fed wire then the impedance will be
very high. Most AM radios have a high impedance input designed to
match a random wire like that. If you use coax, the capacitance of the
coax (center conductor to shield) will essentially short most of the
signal to ground and little will get to the receiver. If you've ever
opened up the coax used on car radios you'll find that it is a special
high impedance coax. A small wire fits loosly thru an insulated tube
with the shield around the outside. This keeps a relatively large
spacing between the center conductor and the shield in order to
minimize capacitance.

Even the car radio type coax would probably have too much capacitance
for the length of run you'll need. Yes, you could put a preamp
(actually an active impedance converter) near the antenna in order to
drive standard 50-ohm coax cable. I classify it as an "impedance
converter" because you probably don't need or want much gain, just the
ability to match the coax impedance. On the broadcast band any gain
will likely increase the noise level as much as the signals.

If you don't have too much noise pick up from the house electrical
wiring, etc you can probably just run a single wire feed line down
into the basement. Keep it as far away as possible from any conductive
material like rain gutters, etc.

In looking at some of the options I was pointed to on this matter, it seems
perhaps the first thing I should do is run a simple wire down into the
basement as best I can and see how that fares. At least now Im armed with
some alternative ideas should that method prove too lossy/noisy.

thanks again to all who responded!

Dave



"Bob Lewis (AA4PB)" wrote in message
...
Even the car radio type coax would probably have too much capacitance
for the length of run you'll need. Yes, you could put a preamp
(actually an active impedance converter) near the antenna in order to
drive standard 50-ohm coax cable. I classify it as an "impedance
converter" because you probably don't need or want much gain, just the
ability to match the coax impedance. On the broadcast band any gain
will likely increase the noise level as much as the signals.

If you don't have too much noise pick up from the house electrical
wiring, etc you can probably just run a single wire feed line down
into the basement. Keep it as far away as possible from any conductive
material like rain gutters, etc.

In looking at some of the options I was pointed to on this matter, it seems
perhaps the first thing I should do is run a simple wire down into the
basement as best I can and see how that fares. At least now Im armed with
some alternative ideas should that method prove too lossy/noisy.

thanks again to all who responded!

Dave



"Bob Lewis (AA4PB)" wrote in message
...
Even the car radio type coax would probably have too much capacitance
for the length of run you'll need. Yes, you could put a preamp
(actually an active impedance converter) near the antenna in order to
drive standard 50-ohm coax cable. I classify it as an "impedance
converter" because you probably don't need or want much gain, just the
ability to match the coax impedance. On the broadcast band any gain
will likely increase the noise level as much as the signals.

If you don't have too much noise pick up from the house electrical
wiring, etc you can probably just run a single wire feed line down
into the basement. Keep it as far away as possible from any conductive
material like rain gutters, etc.

Even the car radio type coax would probably have too much capacitance
for the length of run you'll need. Yes, you could put a preamp
(actually an active impedance converter) near the antenna in order to
drive standard 50-ohm coax cable. I classify it as an "impedance
converter" because you probably don't need or want much gain, just the
ability to match the coax impedance. On the broadcast band any gain
will likely increase the noise level as much as the signals.

If you don't have too much noise pick up from the house electrical
wiring, etc you can probably just run a single wire feed line down
into the basement. Keep it as far away as possible from any conductive
material like rain gutters, etc.